- 100 grams of O 2 (g) and 100 grams of He(g) are in separate containers of equal volume.
Both gases are at 100°C. Which one of the following statements is true?
A. Both gases would have the same pressure.
B. The average kinetic energy of the O 2 molecules is greater than that of the He
molecules.
C. The average kinetic energy of the He molecules is greater than that of the O 2
molecules.
D. There are equal numbers of He molecules and O 2 molecules.
E. The pressure of the He(g) would be greater than that of the O 2 (g).Answer: E
Oxygen gas weighs 32 grams per mole, whereas helium gas weighs only 4 grams per mole.
One can see that there are roughly 3 moles of oxygen molecules and 25 moles of helium mole-
cules. Gas pressure is proportional to the number of molecules and temperature, and inversely
proportional to the size of the container. Since there are more helium molecules, you would ex-
pect a higher pressure in the helium container (with all other variables being held constant). As
long as the temperatures of the two containers are the same, the average kinetic energies of the
two gases are the same.
- Which one of the manometers below represents a gas pressure of 750 mm Hg?
(Atmospheric pressure is 760 mm Hg.)
Answer: D
Manometer (D), the correct answer, shows the air pressure to be 10 mm Hg greater than that of
the gas. Manometer (A) shows the gas pressure to be 10 mm Hg greater than air pressure.
Manometer (B) shows the gas pressure to be equal to that of the air pressure. Manometer (C) is
a closed manometer showing that the pressure of the gas is 10 mm Hg greater than the un-
known pressure on the left side of the manometer. Manometer (E) is also closed and shows that
the gas on the right side (770 mm Hg) is exerting a pressure 10 mm Hg greater than the gas on
the left side.
(^10) mm
(A) (B) (C) (D) (E)
(^10) mm
(^10) mm^10 mm
Part II: Specific Topics